Lithotrophic organisms use inorganic substrate for biosynthesis. These “rock eating” microorganisms living in subsurface environments comprise great proportion of the biomass found on and inside our earth and hot springs represent accessible windows to sample the deep biosphere. Microorganisms isolated from hot springs are known to use hydrogen as an energy source. In this project we will add hydrogen and monitor growth of both microbial cells and viruses during in situ experiments in Yellowstone National Park hot spring sediments. Changes in total cell numbers will be monitored, and total DNA and expressed mRNA will be extracted and sequenced using cutting edge metagenomic techniques. Viruses are known to inhabit hot springs, and they will be studied using transmission electron microscopy and 454 pyrosequencing of DNA from viral communities. The project aims to study bacterial and viral interactions in both batch cultures and continuous cultures to gain knowledge of evolution over time in environments known to inhabit deeply branching microorganisms close to the root of the phylogenetic tree of life. We will study the effect of top down predation by viruses on the regulation of microbial and viral diversity. This proposal lies within the field of environmental / geomicrobiology, where we will study microbial and viral interactions within the environment the organisms are found. The applicant for the fellowship, Hallgerd Eydal holds a PhD in microbiology and has expertise in working with subsurface samples and viral ecology. During this project she will get training in new methods at new locations. The outgoing host is John Spear at Colorado School of Mines in the United States. Spear has ten years experience of working in Yellowstone, and a lab with skills in metagenomic analysis. The returning host is the University of Bergen in Norway with professors Lise Øvreås and Ruth-Anne Sandaa - skilled scientists within microbial diversity and viral ecology, respectively.